Menu and layout bugs in docs

[cnanney]

https://knsv.github.io/mermaid/

Erratic menu behavior, using Chrome 56:

Body text also gets partially covered by menu.

[saveman71]

I'm pretty sure it's been introduced by fb70b97 (at least the overflow problem), since the page is good until the section of usage.md that was modified in this commit
(cc @knsv)

[tylerlong]

Docs has been moved to https://github.com/mermaidjs/mermaid-gitbook